The interaction of a series of isomeric palladium tetrakis(N-methyl-x-pyridinium)porphyrin (x = 2, 3, and 4) with DNA and synthetic polynucleotides has been studied. Both intercalation between base pairs (x = 3 and 4) and electrostatic binding to the external phosphate chain (x = 2) are observed. Once bound to DNA, the excited triplet state of these cationic porphyrins can be quenched by oxygen dissolved in the surrounding aqueous reservoir and by methyl viologen bound to the phosphate chain. Rates’ of quenching can be related to the location of the bound porphyrin on the DNA duplex. Singlet and triplet energy transfer is also observed from intercalated acridine orange to a porphyrin (x = 3) that is intercalated into the same helix. The rates of singlet energy transfer are discussed in terms of competing Forster- and Dexter-type mechanisms, and it is concluded that the DNA duplex does not provide a particularly attractive medium for electron exchange.